The present invention relates to a production controller and, in particular, to a production controller which controls a lot flow of a production line including a plurality of production facility groups so as to control the work start such that the number of lots waiting for the work before the corresponding facility group does not exceed the capacity of a corresponding buffer (keeping shelf) which stores the waiting lots.
In a production system, the work on a lot is carried out by using resources, such as facilities, workers and jigs. These resources have their own working capabilities and are used based on individual operation plans. Lots to be processed are fed daily to the production system, and the production system starts the work on the lots based on the predetermined working procedure using the working resources of the production system.
In the production line comprising a plurality of facility groups, a lot which arrived at the facility group of a certain process and was subjected to the work is transferred to the facility group of the next process. In this case, if the capacity of the facility group where the lot has arrived is sufficient, the work on the lot is immediately started in that facility group. On the other hand, if the capacity of that facility group is not sufficient, the transferred lot queues up before the facility group and waits for the work.
Accordingly, depending on a relationship between the capacity of each of the facility groups and the lot feed plan, a fair number of waiting lots may be waiting before each of the facility groups. For storing these waiting lots, the production line is normally provided with keeping shelves of various sizes, that is, buffers, at the respective facility groups.
In general, the buffer has a limitation in its capacity (kind of working resource). Accordingly, in the production control system, it is necessary to carry out a lot progress control to manage the amount of the process lots so as not to exceed the buffer capacities at all the facility groups.
Conventionally, in some production lines, when the number of the waiting lots is likely to exceed the buffer capacity, provisional keeping shelves are prepared to temporarily accommodate the waiting lots by workers.
However, for example, in a production line for large liquid-crystal panels, since the size of the liquid-crystal panel and further the weight thereof are large, it is difficult for the worker to handle the panels per unit of a lot. Therefore, automation of the production lines has been advanced. Further, in view of idle space or layout of the production line, it is difficult to provide space for provisional keeping shelves or provide large-capacity buffers for keeping a large amount of the lots. Accordingly, it is desired that the work start for a lot at each of the facilities is automatically controlled so that the number of the waiting lots at the facility group of the next process does not exceed the buffer capacity, predetermined per facility group, during the work in the facility or the transfer work.
Various methods have been proposed for automation of the lot flow control of the production line, for example, as described in Japanese Unexamined Patent Publication No. 5-12298 which aims to realize an efficient lot flow for those lots having a high working priority by obtaining data even about lots which are scheduled to reach the facility group of the current process from the facility group of the prior process. On the other hand, for example, Japanese Unexamined Patent Publication No. 7-129672 has proposed a production control method for controlling the actual work progress to follow the target feed amount.
On the other hand, with respect to the lot flow control of the production line considering the buffer capacity, for example, about the system for controlling the work start by observing work waiting lots at the next process facility group, a modeling example of the flow control using a rule-based system is shown in literature (1) (B. R. Tibbitts: "Flexible simulation of a complex semiconductor manufacturing line using a rule-based system", IBM J. RES. DEVELOP. Vol. 37, No. 4, July 1993, pp. 507 to 521). In this flow control model, the flow control of a kanban system is realized between the continuous facility groups by means of (1) a lot start control toward the next process facility group and (2) a lot start control from the prior process facility group.
Further, in literature (2) (Henry D'Angelo, Michael Carmanis, Susan Finger, Anton Marvretic, Yannis A. Phillis, Edward Ramsden: "Event-driven model of unreliable production lines with storage", INT. J. PROD. RES., 1988, Vol. 26, No. 7, 1173-1182) and literature (3) (Vassilis S. Kouikoglou, Yannis A. Phillis: "An Exact Discrete-Event Model and Control Policies for Production Lines with Buffers", IEEE Trans on Automatic Control, Vol. 36, No. 5, 1991, pp. 515 to 527), a capability evaluation technique for the production system under BC control is described. With a view to capability evaluation and without explicitly modeling a lot to be processed, attention is paid to time points of the state changes of the facilities and the buffers, and the number of the process lots in each facility is derived from a workable time of the facility and a facility throughput so as to realize the capability evaluation. Specifically, the control of the buffer level is indirectly realized through adjustment of the facility throughput. Further, calculation of time of buffer block occurrence is carried out in advance based on the throughput, and the throughput adjustment and update of the facility state and the buffer state for buffer level control and work start control are carried out upon occurrence of facility maintenance, failure and buffer block relative to the facility groups prior to and subsequent to the occurrence of the state change, and further, the work lot is derived based on a time segment of the state change and the throughput.
Further, Japanese Unexamined Patent Publication No. 8-179808 (Patent Application No. 6-320310) describes as prior art a flow control using a condition represented by the following formula (1) as a work start criterion for assuring that the number of the waiting lots of the next process facility group does not exceed the buffer capacity.
Assuming that a buffer capacity of a next process facility group RGi+1, while a current process facility group being RGi, is set as Si+1, the number of work waiting lots of RGi+1 at a time t is set as Ni+1(t), a facility group which can be a prior process facility group relative to RGi+1 is set as RGprevOi+1.sup.j, and the number of lots whose next process is Oi+1 among the process lots at RGprevOi+1.sup.j is set as NpreveOi+1.sup.j (t), the formula (1) is given by EQU Si+1.gtoreq.Ni+1(t)+.sub.j .SIGMA.N.sub.prevOi+1.sup.j (t) (1)
Further, the foregoing Japanese Patent Application No. 6-320310 describes as prior art a device for an automatic lot flow control based on the estimated amount of process lots, wherein the number of lots to be accommodated in a buffer is limited.
Further, Japanese Patent Application No. 7-214160 describes a device for a lot flow control based on the estimated amount of process lots, wherein no buffer is provided.
Further, for considering the buffer, the scheduling method shown in the foregoing Japanese Patent Application No. 6-320310 may be combined with the conventional technique shown in the foregoing Japanese Patent Application No. 7-214160.
On the other hand, with respect to scheduling of the production line considering the buffer capacity, in literature (4) (Hisashi Tamaki, Yoshikazu Nishikawa "Modeling of Job-shop Scheduling Problems with In-process Buffer Capacity", Metrology Automobile Institute Thesis, 1995, Vol. 31, No. 7, pp. 933 to 940), the scheduling problem considering the buffer capacity is formulated as a mixed integer programming problem and substituted for the selection graph model, the time Petri net or the Gantt chart model.
In the foregoing literature (4), with respect to the Gantt chart model, a method is referred to, wherein, assuming that a maximum value of a time at which one or more lots are put in an immediately-before buffer is set as t1, a time at which room is caused in an immediately-after buffer is set as t2, and a time at which a facility to be used becomes idle is set as t3 (if undetermined, set to .infin., respectively), scheduling is carried out by deriving an earliest processable time segment defined by an earliest startable time max (t1, t2, t3) satisfying EQU max(t1, t2, t3)&lt;.infin.
and an earliest finishable time.
However, the foregoing conventional lot flow control methods under the finite buffer capacity limitation have the following problems, respectively:
In the conventional production control methods aiming at the automation of the lot flow control of the production line as proposed in the foregoing Japanese Unexamined Patent Publications Nos. 5-12298 and 7-129672, the lot flow control considering the buffer capacity limitation of each of the facility groups is not carried out.
Accordingly, if such a conventional lot flow control is applied to the production line having the finite capacity buffers, it is difficult to suppress an occurrence of the waiting lots within the capacity of each buffer. For this reason, the current state that the lot flow control in the production line does not rely on the automation, but is carried out manually, and thus, the automation of the production control as an expected target can not be realized.
Further, if the conventional kanban system flow control shown in the literature (1) "Flexible simulation of a complex semiconductor manufacturing line using a rule-based system" is applied, since the work start is notified to the prior process after room is caused in the buffer, a time lag corresponding to a work time in the prior process facility group is caused until a lot subjected to the work start in the prior process facility group finally fills the buffer. Particularly, when the work time in the prior process facility is long, the lot work number per unit time is reduced so that the control for smoothly advancing the work becomes difficult.
Further, in the conventional methods shown in the literature (2) "Event-driven model of unreliable production lines with storage" and the literature (3) "An Exact Discrete-Event Model and Control Policies for Production Lines with Buffers", the process model is used wherein one process (facility group) includes one facility. Accordingly, for dealing with the problems of the general production lines where one process (facility group) includes a plurality of facilities in which the throughput differs per facility or the maintenance term differs per facility, it is necessary to adjust the throughput and change the handling of the maintenance term, the failure term and the like per unit of facility group. As a result, these conventional methods can not be used advantageously as they are.
Further, in these conventional methods, no consideration is given to a time for post-arrangement work which does not bind the facility after the work start, and a time for lot transfer to the next process, which are frequently observed in the general production lines. The existence of the post-arrangement work time and the lot transfer time causes a time lag between he throughput adjustment in the facility groups and the actual change in number of lots in buffers. As a results, since the accurate number of the work lots can not be calculated, the proper number of lots in buffer can not be estimated so that the flow control as an expected target can not be realized.
Further, for speeding up the process, the work objects (lots) on the production line are not explicitly modeled, and the number of the work lots is derived through calculations. Thus, attributes (kind, work condition, work priority, and so forth) of the individual lots applied to the actual flow control can not be considered.
There has been a problem that, in the prior art which does not aim at the flow control, the flow control of the work lots as an expected object can not be accomplished.
Further, in the conventional flow control method shown as the prior art in the foregoing Japanese Patent Application No. 6-320310, for satisfying the buffer capacity limitation of the next process, the sum of the number of waiting lots at the next process buffer and the number of lots in process scheduled to arrive at the next process buffer is regarded as a lot feed amount of the next process buffer, and the work start in the current process facility is determined so as to hold this lot feed amount within the next process buffer capacity.
Although this work start determination method assures that the next process buffer capacity is followed, the lot feed amount tends to be estimated greater than the actual lot stay amount in the buffer. For example, in the state that the next process buffer capacity is not greater than the number of facilities of the facility group prior thereto, even if the process capacity of the next process facility group is high so as not to cause a lot stay in the buffer, the number of the lots to be processed in the prior process facility group is limited to not greater than the next process buffer capacity. Thus, although the next process buffer actually has room for a further lot so that the work is executable, there is a high possibility that the work start is reserved.
As a time period from the lot work start in the prior process facility group to the arrival of the lot at the next process facility group becomes longer, a time period (time lag) for which the new work start is suppressed increases so that execution of the lot feed control for smoothly advancing the work becomes difficult.
As a result, the facilities are operated at the production capacity lower than the actual production capacity. Accordingly, at the facility group where a work time is long, the number of lots waiting for the work inevitably increases so that the buffer capacity is reached in due course to reserve even the work in the prior process facility group, and finally, the work progress of the whole system is stagnated.
Further, in the prior art shown in the foregoing Japanese Patent Application No. 6-320310, the lot flow control is realized, wherein the work start control is carried out based on the estimated amount of the process lots in the next process facility group so as to utilize the next process buffer capacity. However, in this prior art, when the number of the facility groups to which the same control is continuously applied in the working procedure becomes not less than three, there is a possibility that the work start can not be estimated subsequent to the third facility group.
For example, it is possible to determined upon the work start in the first facility group based on only the buffer state of the second facility group and the state of the usable facility of the second facility group that the work start can be carried out. Upon the work start in the second facility group, the work start is determined based on the buffer state of the third facility group and the state of the usable facility of the third facility group. Similarly, the work start in the third facility group is determined by the states of the fourth facility group, and the work start in the fourth facility group is determined by the states of the fifth facility group. Accordingly, the work start in the i-th facility group is determined by the states of the (i+1)-th facility group.
As a result, when the facility groups having the finite buffers continue, it is necessary to carry out the work start in the facility groups within the continuous zone by knowing the states of all the facility groups in the continuous zone subsequent to the work start process and satisfying the finite buffer limitation in the respective facility groups.
If the foregoing conventional technique is applied to the production line in which three or more facility groups have the finite buffers continue, since the work start in the second facility group is carried out without considering the states of the third or subsequent buffer, the number of the staying lots exceeds the buffer capacity of the corresponding facility group so that the control following the buffer capacity, as an expected target, can not be accomplished. Specifically, when the work start control is applied to the zone with three or more continuous processes, scheduling for following the buffer capacities in the continuous process zone can not be ensured only based on the conventional work start determination for the facility groups.
Further, when the conventional technique described in the foregoing Japanese Patent Application No. 7-214160 is applied to the flow control of the production line having the buffers, the work start control is carried out without using the buffers of the respective facility groups so that the control following the buffer capacity, as an expected target, can not be accomplished. Moreover, since the work start is suppressed when the scheduling can not be achieved for the facility group to which the control is continuously applied, the number of the work reports is reduced to lower the production efficiency.
Further, if the conventional scheduling method described in the foregoing Japanese Patent Application No. 6-320310 is applied to the device described in the foregoing Japanese Patent Application No. 7-214160, the operation time periods of all the facilities and the buffer capacity are fully utilized in a first generated bottle neck facility group among the continuous process facility groups to which the control is applied.
However, after the occurrence of the bottle neck state, the buffer of the facility group of a process prior to a process of the bottle neck facility group is subjected to change without being utilized, the control following the buffer capacity, as an expected target, can not be accomplished thereby to lower the production efficiency.
If the scheduling shown in the literature (4) "Modeling Technique of Scheduling Problem Considering Buffer Capacity" is applied to the production control, it is necessary, in the conventional method based on the mixed integer programming problem, to solve the mixed integer programming problem per execution of the work in real time, and thus it may be not practical.
In the literature (4), in the conventional technique based on the scheduling method shown in the Gantt chart model, assuming that a maximum value of a time at which one or more lots are put in an immediately-before buffer is set as t1, a time at which room is caused in an immediately-after buffer is set at t2, and a time at which a facility to be used becomes idle is set as t3 (if undetermined, set to .infin., respectively), the scheduling is carried out by deriving an earliest processable time segment defined by an earliest startable time max (t1, t2, t3) satisfying max (t1, t2, t3)&lt;.infin. and an earliest finishable time. Accordingly, the occurrence of the time lag due to the work start control in the continuous facility groups can be avoided. However, since the scheduling is carried out utilizing only the local data, that is, the work start facility group and the next process facility group thereof, when the work start control is applied to the finite buffer facility groups continuing over three or more processes, even if the work start is determined based on the scheduling method in which the earliest processable time segment is derived, the scheduling for following the buffer capacities in the continuous processes can not be accomplished as in the foregoing Japanese Patent Application No. 6-320310 so that the control following the buffer capacity, as an expected target, can not be realized.
In many of the foregoing conventional techniques, the work start control is carried out only by the local data about the prior and subsequent process facility groups. Accordingly, for example, if a chain of the work start suppression occurs due to the work start control, it is essentially difficult to escape from the situation of the work complete stop or the like due to a deadlock of the work progress. Specifically, in many of the conventional techniques carrying out the work start control based on the local data, the work start control based on the large-regional data over the facility groups of several processes necessary for avoiding the deadlock is not carried out so that the deadlock state of the work progress tends to be caused.